Materials Science Forum Vols. 539-543

Abstract: Hypereutectic prealloyed Al-20wt.%Si powders were prepared by the gas atomization process. Characteristics of the atomized Al-Si powders were investigated. With increasing gas pressure of the atomization, the average powder size and oxygen content decreased. Primary Si particles in the as-atomized powders were about 8-10 ㎛. Sintering behavior of the atomized Al-Si powders was discussed with sintering temperature. As-atomized powders were hot-pressed and extruded into a cylindrical shape. 96-99% of theoretical density was achieved by the hot consolidation. Microstructure changes as well as hardness and tensile properties of the samples were evaluated in this paper.

Abstract: A 3-D coupled temperature-displacement finite element analysis is performed to study an ultrasonic consolidation process. Results show that ultrasonic wave is effective in causing deformation in aluminum foils. Ultrasonic vibration leads to an oscillating stress field. The oscillation of stress in substrate lags behind the ultrasonic vibration by about 0.1 cycle of ultrasonic wave. The upper foil, which is in contact with the substrate, has the most severe deformation. The substrate undergoes little deformation. Apparent material softening by ultrasonic wave, which is of great concern for decades, is successfully simulated. The higher the friction coefficient, the more obvious the apparent material softening effect.

Abstract: The two-particle model describes the approach of particle centres and the growth of the interparticle contacts during sintering of metal powders. Unfortunately the comprehensive description of processes inside of three dimensional specimens must consider the contribution of particle rearrangements. The recent developments of combined micro focus computed tomography (CT) and 3D photogrammetric image analyzing give the opportunity to obtain the experimental data required to overcome the shortcomings of sintering theories based on the two-particle model. The analysis of spherical poly and single crystalline copper powder was performed by CT. In addition a single crystal specimen was analyzed by high resolution synchrotron radiation tomography - a more sophisticated analysis method with very limited availability. The analysis of the 3D tomographic image by photogrammetric image analyzing yielded the positions and radii of all particles and their contact partners as well. A statistical analysis of the retrieved data was performed. The formation and breaking of necks during sintering could be observed. An in-depth analysis of the particle rotation with respect to the coordination number and local density will be presented.

Abstract: Mechanical alloying (MA) is one of the most appropriate severe plastic deformation processes applied to powders in order to obtain good mixing, new different systems or alloys and / or to reduce particles grain size from the starting powders. In the present work, MA was used to obtain prealloyed powders of Cu-Fe base alloys suitable for subsequent sintering. Mechanical alloying process parameters have been optimised to reduce container and balls contamination in dry and inert atmosphere conditions. For safe final powder manipulation, the final aggregate size requirement needed to be kept in the microscopic scale. The microstructural results showed that the components of the original powders were intimately combined resulting in alloyed aggregates suitable for sintering and for mixing to other hard materials to obtain composites. The materials selection for container and balls is critical in order to avoid contamination. In the present work, no contamination was detected in the final processed particles. Consolidation of these powders was carried out as well as calorimetric tests for studying their stability.

Abstract: For a comprehensive understanding of the direct metal laser sintering (DMLS) process and for the successful introduction of this technology, some investigations related to the characteristics of the powders and the individual sintered layers were therefore performed. Also possibilities of hard coatings deposition for further improvements the wear and temperature resistance of tool inserts, and investigations particularly focused into the industrial applications of DMLS tooling inserts are presented.